Wireless terminal devices such as laptop personal computers, tablet terminal devices, and smartphones are equipped with wireless communication devices for wireless WAN, wireless LAN, etc. Electromagnetic waves radiated from an antenna may have adverse effects on a human body in proximity to the antenna. The Federal Communications Commission (FCC) of the U.S. Government and the pertinent organizations in other countries have set permitted levels of electromagnetic energy that is allowed to be absorbed into human beings, given in terms of a unit referred to as the specific absorption rate (SAR). The SAR is a measure of the amount of electromagnetic energy absorbed in a human body per unit time per unit mass.
In the United States, Canada, Taiwan and the like, the SAR limit with respect to local exposure of a human body is set to 1.6 W/kg averaged over 1 gram of living tissue. In several countries in Europe and Oceania, the SAR limit is set to 2.0 W/kg averaged over 10 grams of living tissue. Presently, there are countries that have no specific SAR limits in place. In order to meet the SAR standards set in various countries, some wireless terminal devices are equipped with a so-called dynamic power reduction (DPR) function which, when detecting that a human body is in proximity to an antenna, automatically reduces the electromagnetic energy radiated from the antenna.
Japanese Patent Application Laid-Open No. 2003-209483 shows a technique for reducing the SAR in a mobile communication device. When a sensor senses that the mobile communication device is in proximity to a human body, a controller reduces at least one of the average power and the duration of the radio frequency electromagnetic energy radiated by the mobile communication device. Japanese Patent Application Laid-Open No. 2006-340180 describes a method for controlling the SAR in a portable terminal device equipped with a plurality of antennas, while suppressing degradation of communication quality. In the wireless terminal device, the antenna to be disabled is determined by detecting the display orientation, the holding orientation, or the installation status.
Japanese Patent Application Laid-Open No. 2010-045698 describes a tablet computer which stops transmission from an antenna when the approach of a human body is detected on the basis of the reception power of a spurious signal, in order to clear the SAR-related problems. RF Exposure Procedures Update, Federal Communications Commission, OET Laboratory Division TCB workshop online publication, October 2010, describes an SAR measurement method taking DPR into consideration is available on the Internet at the following URL:http://transition.fcc.gov/oet/ea/presentations/files/oct10/SAR_RF_Exposure_Procedures_Updates_101910-KC.pdf.